1. Field of the Invention
This invention relates to a signal output circuit ideal for an interface between a circuit and a device, particularly a parallel interface used widely in a peripheral apparatus such as a printer, a parallel interface circuit and a printer apparatus which uses this parallel interface circuit.
2. Description of the Related Art
It known widely that a parallel interface in conformity with the Centronics standard (this shall be referred to as a "Centronics interface" below) is used to transmit data between a host device such as a personal computer and a peripheral device such as a printer. An open collector circuit is used as the signal output circuit of the Centronics interface.
Since the Centronics interface is designed only for data communication in one direction, namely from the host device to the peripheral device, a bidirectional parallel interface standard having upward compatibility with the Centronics interface has been stipulated by the IEEE. This standard is referred to as "IEEE Std. 1284-1994: Standard Signaling Method for Bi-directional Parallel Peripheral Interface for Personal Computer", or the so-called "IEEE 1284".
The communication modes stipulated by IEEE 1284 include an ECP mode capable of dealing with communication speeds up to a maximum of about 2 megabytes per second. This means that the conventional open collector circuit is too slow to be used as the signal output circuit. This makes it necessary to use a totem pole circuit.
FIG. 10 shows an example of a circuit diagram illustrating a totem pole circuit. If a transistor Tr3 in the totem pole circuit is turned on, transistor Tr1 turn off and the transistor Tr2 turns on. If the transistor Tr3 is turned off, then the transistors Tr1 turns on and the transistor Tr2 turns off. Accordingly, a low-level output is obtained if the transistor Tr3 is turned on and a high-level output if the transistor Tr3 is turned off.
If the signal output circuit is made the totem poll circuit, however, the following problems not seen in the open collector circuit arise:
(1) If the output signal is accidentally left short-circuited to ground or to another output signal for an extended period of time, the elements constituting the signal output circuit will deteriorate or be destroyed. Though such an accident may be prevented by turning off the power supply of the device when a connecting cable is installed, instances in which the operator forgets to turn off power at such time frequently occur. Another conceivable accident is the short-circuiting of output signals within the connecting cable due to aging of the cable.
(2) Data signals are bi-directional in the ECP mode. Consequently, if the data signal is left in the output state in both the host and peripheral devices, the elements of the signal output circuit may sustain deterioration or damage.
There is a possibility that situations such as these may arise when directional control of the data signal fails owing to an event such as runaway of the CPU within the device.